Proof tested just means it's OK to use smokeless powder in the gun. It's similar to "Nitro" tested.
It's assumed that a firearm made today is to be used with smokeless powder, so you don't see that as much on new gun. I don't think black powder cartridges were being widely used in the 50's but probably a good amount of firearms in existence at the time were not proof tested.
I can't say about the steel shot. I have been told that steel shot scores the inside of the barrel, but I don't have any data to back that claim up. I have shot steel through my Benelli M1-S90 on several occasions (with steel OK'd chokes) and the barrel is still fine.
I would play it safe until you find out for sure. Steel is very different from lead in that it won't deform quite as readily, so it may have problems squeezing through the full choke.